Since their invention, integrated circuits have incrased greatly in complexity and today have many more components than they had initially. As the number of components in the circuits continues to increase and the dimension of the individual components of the circuits continue to decrease in size, it becomes increasingly difficult to fabricate integrated circuit chips with every component having satisfactory characteristics. That is, as is well known, a plurality of chips are fabricated on a single wafer, and each chip has an integrated circuit with numerous individual components. However, any individual integrated circuit may have one or more defective components which render the entire integrated circuit commerically useless. Therefore, the yield of useful chips per wafer will decrease as the number of defective components increase, and this is obviously undesirable because a low yield increases the cost of an integrated circuit.
Therefore, techniques have been sought which increase the yield of integratecd circuits. One technique increases the yield by both refining and better controlling the processing techniques. Another technique includes redundant components on the chip. The redundant components are either systematically included or excluded in the integrated circuit as defective components are found. The redundant components are typically included or excluded from the integrated circuit by completing or breaking an electrical circuit and thus repairing the circuit. This repair is frequently done by using radiation, e.g. electromagnetic energy, to break a conductive link. The link is simply a conductive, e.g. aluminum or polysilicon, line or runner on a substrate. The energy is typically supplied by a laser in amounts sufficient to melt or vaporize the desired portion of the link. The repair process, to be successful in a complex circuit, might involve the repair of hundreds of links. While repair is generally practiced only for complex circuits, simpler circuits often have their fabrication completed by selective inclusion or exclusion of individual components to yield a custom integrated circuit. This may involve tens of thousands of links. For both repair and customization, it is desirable that the breaking of the links be done consistently.
Although conceptually simple, the use of a laser to break links encounters difficulties in practice thus making its use more difficult than might be thought initially. The amount of energy used must be sufficient to consistently destroy the links, but it must also be less than the amount of energy which would damage either the underlying or surrounding material. There is thus a range of allowable energies. Remaining within this range is complicated by variations in the physical characteristics of the link structure as well as variations in the output energy of the laser and the targeting accuracy of the laser.
Techniques have been developed to study the effects of laser radiation on metal and semiconductor surfaces. Perhaps the first technique developed studied the temporal change in intensity of light reflected from the surface during irradiation. Such a technique is called time resolved reflectivity and has been used since Sooy et al. published an article in Applied Physics Letters, 5, pp. 54-56, Aug. 1, 1964. Sooy was interested in using a semiconductor as a Q switch, and the reflectivity from a semiconductor surface determined its utility as a cavity mirror. Sooy attributed the time dependence of the reflectivity as being due to increased carrier concentration and melting of the semiconductor surface caused by the radiation. Birnbaum and Stocker reported the reflectivity of semiconductors in Journal of Applied Physics, 39, pp. 6032-6036, December 1968. They found an enhanced reflectivity from semiconductor surfaces which they attributed to a liquid layer, formed by semiconductor melting, on the surface having matallic properties. They also studied reflectivity from several metals and found a decrease in reflectivity which they attributed to damage on the metal surface.
Both of these studies used a two laser experimental arrangement in which one laser acted as the pump and the other laser acted as the probe. That is, one laser, i.e., pump, produced a response in the target material, and the production of this response was monitored by measuring the intensity of the reflected beam from the other, i.e., probe, laser. Two beam techniques are difficult to implement for link blowing because the probe beam and the pump beam must overlap; this is difficult to accomplish due to the small beam and feature sizes.
A single laser study of ion-implanted silicon was reported by Liu et al. in Applied Physics Letters, 34, pp. 363-365, Mar. 15, 1979. The precise experimental setup is not described in detail, but the article states that the samples were irradiated at approximately normal incidence, and that both the incident and reflected pulses were detected separately with photodiodes. It was found that, as the incident energy increased, the melting onset for an amorphous to liquid transition moved to an earlier part of the pulse, i.e., the transition occurred earlier in time. The authors also found an enhanced reflectivity which was sttributed to the presence of melted silicon. While the study was interesting technically, Liu et al. was interested in studying the laser annealing of ion implated silicon; e.e., the liquid phase epitaxial regrowth of silicon after melting, and the study did not discuss laser irradiation of multilevel structures such as those present when links are broken. None of the studies discussed examined any phenomena comparable to link explosion.
There are teachings in the literature of techniques useful in link blowing. For example, U.S. Pat. No. 4,853,758, issued on Aug. 1, 1989, to Frederick S. Fisher describes an exemplary technique for using a laser to blow links with a wide range of acceptable energies. In an exemplary embodiment, the technique places links on upper levels on dielectric pedestal whereby the links on lower levels have the thickness of the overlying dielectric reduced. The reduced dielectric thickness reduces the minimum laser energy required to cleanly blow the links on both levels.
However, it would be desirable to have a process that incorporates temporal monitoring of the link blowin process.